If you dig down far enough you should be able to find as much lava as you need. The deeper you go the more common lava is (statistically speaking). You will also sometimes find obsidian where lava and water met naturally, but this isn't as desirable as raw lava. Either way, lava is the only thing that produces obsidian, and unlike with water you can't make Newton cry by creating an infinite pool of it. Lava is commonplace in Nether, however (there are entire seas of the stuff), so you only need to worry about gathering enough for the first portal.

Forget about mining obsidian, that takes forever; the way to do it is to collect the lava with buckets, then build a mould at the portal site, first pouring in a bucket of lava and then one of water, repeating and adjusting the mould shape as needed until the whole thing is complete. You can save four blocks by leaving the outside corners off (which can later be filled in with whatever other blocks you want). You'll need ten buckets of lava and one of water for each portal (water is needed for each block but can can be picked up again and thus recycled), or fourteen if you're not cutting off the corners.

Another detail worth noting is that portals are double-sided (entry from either side works, and you can in fact walk straight through without being teleported), so you could take advantage of this to provide access to a single portal from two different rooms in your fortress or whatever.